litmus,mixture of coloured organic compounds obtained from several species of lichen lichens that grow in The the Netherlands, particularly Lecanora tartarea and Roccella tinctorum. Litmus turns red in acidic solutions and blue in alkaline solutions and is the oldest and most commonly used indicator of whether a substance is an acid or a base.

Treatment of the lichens with ammonia, potash, and lime in the presence of air produces the various coloured components of litmus. By 1840 litmus had been partially separated into several substances named azolitmin, erythrolitmin, spaniolitmin, and erythrolein. These are apparently mixtures of closely related compounds that were identified in 1961 as derivatives of the heterocyclic compound phenoxazine.

Archil (orchil, or orseille) is a mixture of dyes similar to litmus that are obtained from the same lichens by a different method. The manufacture of archil, which produces a violet shade on wool or silk, was introduced into Europe from the Orient about 1300.